A known constant velocity universal joint is disclosed in Japanese Patent Laid-Open Publication No. 2004-125175. A substantially equal constant velocity universal joint to that disclosed in Japanese Patent Laid-Open Publication No. 2004-125175 is shown in the attached drawings, i.e. FIGS. 9 to 11. The constant velocity universal joint includes an outer joint member 1 having a cylindrical body 1a whose inner periphery is formed with three guide grooves 2 at even intervals which extend in parallel with a central axis of the outer joint member 1, an inner joint member 3 having three tripod shafts 3b extending radially outwardly at even intervals from a boss portion 3a, a tip end portion of each tripod shaft 3b being formed with a tripod spherical face 3c, and three roller units 6 for each connecting the outer joint member 1 and the inner joint member 3 to each other. The roller unit 6 includes an annular outer roller 7 having an outer peripheral face 7a of a protruding arc shape that is made contact with a guide face 2a of a large recess arc shape of the guide grove 2 in a rolling manner, and an inner ring 8 engaging with an inner periphery of the outer roller 7 so as to be relatively rotatable thereto by means of a needle roller 9 and having an inner peripheral face 8a into which the tripod spherical face 3c of the inner joint member 3 is rotatably inserted.
According to the constant velocity universal joint disclosed, as shown in FIG. 10 illustrating a main portion of a cross section perpendicular to a central axis of the inner joint member 3, under a torque transmitted state, the outer peripheral face 7a of the outer roller 7 is made contact with and pressed against the guide face 2a of the guide groove 2 by means of force Fe from the tripod shaft 3b on a loaded side S1 on which a driving force is transmitted. On the other hand, on a non-loaded side S2, the outer peripheral face 7a of the outer roller 7 is separated from the guide face 2a of the guide groove 2 so as to form some gap therebetween. In such circumstances, the roller unit 6 is rotatable relative to an axis (which is normally positioned on a center plane Pa formed by connecting respective centerlines of the guide faces 2a on both sides of the guide groove 2) that passes through a center Oa of an arc-shaped cross section of the outer peripheral face 7a of the outer roller 7 on the loaded side S1 and is in parallel with a central axis of the inner joint member 3 (hereinafter simply called “relative to the center Oa”). According to such the constant velocity universal joint of a tripod type, when the joint members 1 and 3 are rotated with a joint angle formed therebetween, the tripod shaft 3b on which the tripod spherical face 3c is formed reciprocates, in response to a rotation of the constant velocity universal joint, in a radial direction relative to the roller unit 6, whose radial position is specified by means of the guide face 2a of the outer joint member 1. Then, a variable offset Lt is generated between a line of action of the force Fe passing through a contact point Q between the tripod spherical face 3c and the inner peripheral face 8a, and the center Oa. In a state shown in FIG. 10, moment for rotating the roller unit 6 relative to the center Oa is generated by means of the force Fe and the offset Lt. Further, friction force μFe (μ is a coefficient of friction between the inner peripheral face 8a and the tripod spherical face 3c) is generated at the contact point Q between the inner peripheral face 8a of the inner ring 8 and the tripod spherical face 3c shifted in a radial direction in the aforementioned manner. A distance between a line of action of the friction force μFe and the center Oa is Lk1. The friction force μFe and the distance Lk1 also generate the moment for rotating the roller unit 6 relative to the center Oa.
The roller unit 6 is rotated relative to the center Oa by means of moment Mz obtained by the force Fe and the offset Lt, and by the friction force μFe and the distance Lk1. In cases where a gap between the outer peripheral face 7a of the outer roller 7 and the guide face 2a of the guide groove 2 on the non-loaded side S2 is small, the outer peripheral face 7a and the guide face 2a are made contact with each other, thereby generating reaction force Fc1 as shown in FIG. 11. Then, thrust force for pushing the outer joint member 1 and the inner joint member 3 in axially opposite directions to each other is induced by means of friction resistance caused by the reaction force Fc1 on the contact portion. According to the moment Mz by the friction force μFe and the distance Lk1, the sign of value is changed depending on a direction of reciprocation of the tripod shaft 3b. However, the absolute value is not changed, which results in a constant absolute value of the induced thrust force. On the other hand, the moment Mz obtained by the force μFe and the offset Lt increases in the quadratic function since the offset Lt increases in the quadratic function in response to a joint angle between the joint members 1 and 3. According to the constant velocity universal joint shown in FIGS. 9 to 11, a diameter of the protruding arc forming the guide face 2a of the guide groove 2 is larger than a height (i.e. a length in a vertical direction in FIGS. 10 and 11) of the guide face 2a. Thus, as shown in FIG. 11, an inclination angle θa of a line of action of the reaction force Fc1 relative to the center plane Pa is small, which leads to a small distance Lc1 between the line of action of the reaction force Fc1 and the center Oa. As a result, the value of the reaction force Fc1 becomes large. In such circumstances, the friction resistance in association with the contact between the outer peripheral face 7a and the guide face 2a is large, which may cause a large induced thrust force. The possibility of vibration and noise may be increased depending on a use condition of the constant velocity universal joint.
Thus, a need exists for a constant velocity universal joint in which an engaging portion between a guide groove and a roller unit is improved so as not to cause vibration or noise.